The aim of this thesis is to develop a better understanding about the degradation mechanisms occurring within lithium-ion cells which eventually lead to their failure. An introduction to the components and operation of Li-ion cells is followed by proposed degradation mechanisms which limit the lifetime of cells. These mechanisms and how they can be identified from electrochemical testing are discussed. Electrolyte additives can be used to improve the safety of Li-ion cells or decrease the rate of cell degradation. Different types of additives and testing methods are discussed followed by an introduction to high precision coulometry which can be used to detect the impact of additives on cycling performance. The High Precision Charger that was constructed for this project is described and shown to meet the desired precision. The use of additives and different materials to extend lifetime of cells is shown to be detectable through the use of high precision coulometry. High precision coulometry proves to be a more efficient way of estimating the lifetime of cells under realistic conditions in a reasonably short amount of time. / MSc. Thesis

Nickel-manganese-cobalt oxide (NMC) cathode materials have been applied in most Li-ion batteries, but there are nevertheless some concerns regarding the stability of this material. High voltage and high temperature during charging have been shown to accelerate the dissolution of NMC due to the release of more acidic components because of rapid electrolyte decomposition. Mn-contaminants (Mn2+) are hypothesized to diminish the diffusion coefficient of Li+ in the electrolyte attributed to the competitive interaction between Mn2+ ions and Li+ ions. With characterizations including 7Li and 1H pulsed field-gradient nuclear magnetic resonance (PFG-NMR) spectroscopy, we demonstrated the Mn (II)-contaminants effect on diffusion coefficient on Li+ dynamics. Under the influence of deliberate manganese salt-additive to the electrolyte, the coin cell shows a capacity fading and unstable charging behavior. The PFG-NMR measurements also validated our hypotheses, as the results showing that Mn-containment causes decrease ~15% in the diffusion coefficient on Li-self diffusion. The activation energy for lithium-ion transport over the temperature range of (273 K - 303 K), was not changed by the presence of the Mn-contaminant electrolyte, which indicates the Mn (II) does not affect the Li-ion transport mechanism. The relative test also includes comparisons with other contamination, such as iron contamination from stain-less steels spacers and copper contamination from the current collector. Additionally, the lithium self-diffusion coefficient was tested before and after charging using a full battery configuration. In electrolytes containing manganese contaminants, a more significant decrease in the diffusion coefficient was observed after charging. Ideally, operando experiments can be used to observe the impact of manganese ions on the SEI. By combining both types of experiments, a closer approximation to the actual application conditions of market-used batteries can be achieved. / Thesis / Master of Science (MSc) / The increasing maturity of lithium battery technology has also promoted the advancement of the electric vehicle manufacturing industry. As an excellent new energy material, the application and development of lithium batteries will be the main trend in the future. However, while improving battery capacity and energy density, lithium batteries also face many challenges. The entire thesis work discusses how electrolyte degradation at high temperatures and high voltages accelerates the dissolution of transition metal manganese ions in NMC materials. The dissolution of manganese ions into the electrolyte creates a competitive effect with lithium ions, thereby reducing the performance of lithium batteries. Here, NMR technology was used to measure the negative effect of manganese ions on the self-diffusion coefficient of lithium ions in the electrolyte. Additionally, a set of operando experiments conducted at different discharge rates demonstrated the changes in mossy lithium and the solid electrolyte interface during the charge and discharge phases caused by pulse discharge. This also proved that such experimental designs can track the impact of manganese ions on the solid electrolyte interface and test the dissolution behavior and impact of manganese ions under different charge and discharge rates.

This thesis studies the self-discharge performance of recently developed rechargeable hybrid aqueous batteries, using LiMn2O4 as a cathode and Zinc as an anode. It is shown through a variety of electrochemical and ex-situ analytical techniques that many parts of the composite cathode play important roles on the self-discharge of the battery. It was determined that the current collector must be passive towards corrosion, and polyethylene was identified as the best option for this application. The effect of amount and type of conductive agent was also investigated, with low surface area carbonaceous material giving best performances. It was also shown that the state of charge has strong effects on the extension of self-discharge. More importantly, this study shows that the self-discharge mechanism in the ReHAB system involves the cathode active material and contains a reversible and an irreversible part. The reversible portion is predominant and is due to lithium re-intercalation into the LiMn2O4 spinel framework, and results from Zn dissolution into the electrolyte, which drives the Li+ ions out of the solution. The irreversible portion of the self-discharge occurs as a result of the decomposition of the LiMn2O4 material in the presence of the acidic electrolyte, and is much less extensive than the reversible process.

Li-ion batteries have a huge potential for use in electrification of the transportation sector. The major challenge to be met is the limited energy storage capacity of the battery pack: both the amount of energy which can be stored within the space available in the vehicle (defining its range), and the aging of the individual battery cells (determining how long a whole pack can deliver sufficient energy and power to drive the vehicle). This thesis aims to increase our knowledge and understanding of structural changes induced by aging and usage of the Li-ion battery materials involved. Aging processes have been studied in commercial-size Li-ion cells with two different chemistries. LiFePO4/graphite cells were aged under different conditions, and thereafter examined at different points along the electrodes by post mortem characterisation using SEM, XPS, XRD and electrochemical characterization in half-cells. The results revealed large differences in degradation behaviour under different aging conditions and in different regions of the same cell. The aging of LiMn2O4-LiCoO2/Li4Ti5O12 cells was studied under two different aging conditions. Post mortem analysis revealed a high degree of Mn/Co mixing within individual particles of the LiMn2O4-LiCoO2 composite electrode. Structural changes induced by lithium insertion were studied in two negative electrode materials: in Li0.5Ni0.25TiOPO4 using in situ XRD, and in Ni0.5TiOPO4 using EXAFS, XANES and HAXPES. It was shown that Li0.5Ni0.25TiOPO4 lost most of its long-range-order during lithiation, and that both Ni and Ti were involved in the charge compensation mechanism during lithiation/delithiation of Ni0.5TiOPO4, with small clusters of metal-like Ni forming during lithiation. Finally, in situ XRD studies were also made of the reaction pathways to form LiFeSO4F from two sets of reactants: either FeSO4·H2O and LiF, or Li2SO4 and FeF2. During the heat treatment, Li2SO4 and FeF2 react to form FeSO4·H2O and LiF in a first step. In a second step LiFeSO4F is formed. This underlines the importance of the structural similarities between LiFeSO4F and FeSO4·H2O in the formation process of LiFeSO4F.

Multiwall carbon nanotubes (CNTs) were synthesized by catalytic chemical vapor deposition of acetylene and used as templates to prepare CNT-Sb and CNT-SnSb₀.₅ nanocomposites via the chemical reduction of SnCl₂ and SbCl₃ precursors. SEM and TEM imagings show that the Sb and SnSb₀.₅ particles were uniformly dispersed in the CNT web and on the outside surface of CNTs. These CNT-metal composites are active anode materials for lithium ion batteries, showing improved cyclability compared to unsupported Sb and SnSb particles; and higher reversible specific capacities than CNTs. The improvement in cyclability may be attributed to the nanoscale dimensions of the metal particles and CNT’s role as a buffer in containing the mechanical stress arising from the volume changes in electrochemical lithium insertion and extraction reactions. / Singapore-MIT Alliance (SMA)

Polymer nanocomposites are used in a variety of applications due to their good mechanical properties. Specifically, better performance of lithium ion batteries and thermal interface material can be obtained by using conductive materials and polymer composites. In the case of lithium ion batteries, electrochemical properties of batteries can be improved by adding conductive additives and conducting polymer into the cathode. Several samples, to which different conductive additives and conducting polymer were added, were prepared and their electrical resistance and discharge capacity measured. In the thermal interface material case, also, thermal properties can be enhanced by polymer nanocomposites. In order to confirm the thermal conductivity enhancement, samples were synthesized using different filler, polymer and methods, and their thermal conductivity measured. The influence of polymer nanocomposites and results are discussed and future plan are presented. In addition, reasons of thermal conductivity changing in each case are discussed.

Li-ion batteries are widely used in electronics and automotives. Despite their success, improvements in cost, safety, cycle life, and energy density are necessary. One way to enhance the energy density is to find advanced cathodes such as Li-rich layered oxides, which are similar to the commonly layered oxide cathodes (e.g., LiCoO2), except there are additional Li ions in the transition-metal layer, due to their higher charge-storage capacity. Another way of advancing is to design new battery chemistries, such as those involving multivalent-ion systems (e.g., Mg2+ and Zn2+) as they could offer higher charge-storage capacities and/or cost advantages. Li-rich layered oxides have a complex first charge-discharge cycle, which affects their other electrochemical properties. Ru doping was expected to improve the performance of Li-rich layered oxides due to its electroactivity and overlap of the Ru4+/5+:4d band with the O2-:2p band, but it unexpectedly decreased the capacity due to the reduction in oxygen loss behavior. Preliminary evidence points to the formation of Ru-Ru dimers, which raises the Ru4+/5+:4d band, as the cause of this behavior. Li-rich layered oxides suffer from declining operating voltage during cycling, and it is a huge challenge to employ them in practical cells. Raising the Ni oxidation state was found to reduce the voltage decay and improve the cyclability; however, it also decreased the discharge capacity. Increasing the Ni oxidation state minimized the formation of Mn3+ ions during discharge and Mn dissolution, which led to the improvements in voltage decay and cyclability. Extraction of lithium from spinel oxides such as LiMn2O4 with acid was found to follow a Mn3+ disproportionation mechanism and depend on the Mn3+ content. Other common dopants like Cr3+, Fe3+, Co3+, or Ni2+/3+ did not disproportionate, and no ion-exchange of Li+ with H+ occurred in the tetrahedral sites of the spinel oxides. Extraction with acid of Mg and Zn from spinel oxides, such as MgMn2O4 and ZnMn2O4, were also found to follow the same mechanism as Li-spinels. The Mg-spinels, however, do experience ion exchange when Mg ions are in the octahedral sites. Chemical extraction of Mg or Zn with an oxidizing agent NO2BF4 in acetonitrile medium, however, failed due to the electrostatic repulsion felt by the migrating divalent ions. In contrast, extraction with acid was successful as Mn dissolution from the lattice opened up favorable pathways for extraction. / text

Lithium-ion (Li-ion) batteries are critically important for portable electronics, electric vehicles, and grid-level energy storage. The development of next-generation Li-ion batteries requires high-capacity electrodes with a long cycle life. However, the high capacity of Li storage is usually accompanied by large volume changes, dramatic morphological evolution, and mechanical failures in the electrodes during charge and discharge cycling. To understand the degradation of electrodes and resulting loss of capacity, this thesis aims to develop mechanistic-based models for predicting the chemo-mechanical processes of lithiation and delithiation in high-capacity electrode materials. To this end, we develop both continuum and atomistic models that simulate mass transport, interface reaction, phase and microstructural evolution, stress generation and damage accumulation through crack or void formation in the electrodes. The modeling studies are tightly coupled with in-situ transmission electron microscopy (TEM) experiments to gain unprecedented mechanistic insights into electrochemically-driven structural evolution and damage processes in high-capacity electrodes. Our models are successfully applied to the study of the two-phase lithiation and associated stress generation in both crystalline and amorphous silicon anodes, which have the highest known theoretical charge capacity, as well as the lithiation/sodiation-induced structural changes and mechanical failures in silicon-based multilayer electrodes. The modeling studies have uncovered unexpected electrochemical reaction mechanisms and revealed novel failure modes in silicon-based nanostructured anodes. Our modeling research provides insights into how to mitigate electrode degradation and enhance capacity retention in Li-ion batteries. More broadly, our work has implications for the design of nanostructured electrodes in next-generation energy storage systems.

This thesis investigates the mechanical behavior of electrodes in Li-ion batteries. Each electrode in a Li-ion battery consists of host atoms and guest atoms (Li atoms). The host atoms form a framework, into which Li atoms are inserted via chemical reactions. During charge and discharge, the amount of Li in the electrode varies substantially, and the host framework deforms. The deformation induces in an electrode a field of stress, which may lead to fracture or morphological change. Such mechanical degradation over lithiation cycles can cause the capacity to fade substantially in a commercial battery. We study fracture of elastic electrodes caused by fast charging using a combination of diffusion kinetics and fracture mechanics. A theory is outlined to investigate how material properties, electrode particle size, and charging rate affect fracture of electrodes in Li-ion batteries. We model an inelastic host of Li by considering diffusion, elastic-plastic deformation, and fracture. The model shows that fracture is averted for a small and soft host—an inelastic host of a small feature size and low yield strength. We present a model of concurrent reaction and plasticity during lithiation of crystalline silicon electrodes. It accounts for observed lithiated silicon of anisotropic morphologies. We further explore the microscopic deformation mechanism of lithiated silicon based on first-principles calculations. We attribute to the microscopic mechanism of large plastic deformation to continuous Li-assisted breaking and reforming of Si-Si bonds. In addition, we model the evolution of the biaxial stress in an amorphous Si thin film electrode during lithiation cycle. We find that both the atomic insertion driven by the chemomechanical load and plasticity driven by the mechanical load contribute to reactive flow of lithiated silicon. In such concurrent process, the lithiation reaction promotes plastic deformation by lowering the stress needed to flow. Li-ion battery is an emerging field that couples electrochemistry and mechanics. This thesis aims to understand the deformation mechanism, stresses and fracture associated with the lithiation reaction in Li-ion batteries, and hopes to provide insight on the generic phenomenon that involves interactive chemical reactions and mechanics. / Engineering and Applied Sciences

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